White ink for ink-jet

ABSTRACT

To provide an ink-jet white ink of active energy ray-curable type, wherein the white ink has high printing stability and good hiding property, and is particularly suitable for printing on the single-pass ink-jet printing system. An ink-jet white ink of active energy ray-curable type, comprising titanium oxide, a pigment dispersant, and a polymerizable compound, wherein the content of the titanium oxide is from 15 to 30% by weight with reference to the total weight of the ink, and wherein the weight average particle diameter of the titanium oxide is 180 to 250 nm. In an embodiment of the ink, the titanium oxide is titanium oxide that surface is treated with silica and the pigment dispersant comprises a basic dispersant.

TECHNICAL FIELD

The present invention relates to an ink-jet white ink of active energyray-curable type, wherein the white ink has high printing stability andgood hiding property, and is particularly suitable for printing on thesingle-pass ink-jet printing system.

BACKGROUND ART

Ink-jet printing system jets and lands minute droplets of ink fromink-jet heads on a base material to be printed, thereby printing byforming an image or characters on the base material. This ink-jetprinting system does not require a printing plate for printing.Electrographic system is a widely known as a typical printing systemrequiring no printing plate. However, the ink-jet printing system issuperior to the electrographic system in the initial cost of theapparatus, the running cost of printing, the apparatus size, andhigh-speed printability.

The ink-jet printing system typically uses ink-jet ink of active energyray-curable type. This ink mainly contains polymerizable compounds suchas a reactive monomer and oligomer, and a polymerization initiator suchas a photo-radical generator and a photoacid generator. In the printingon this printing system, the ink is landed on a base material to beprinted, and then active energy rays are applied to the ink to causepolymerization reaction of the ink components, thereby forming an imageand characters.

In recent years, with the improvement of performance of ink-jet heads,the application of the ink-jet printing system to the existing printingmarket using offset printing and other systems is expected. In theexisting printing market, productivity is very important. However, themulti-pass printing system is used in the sign market, and this printingsystem is insufficient to achieve the desired productivity. Therefore,in the printing market, in order to achieve productivity using theink-jet printing system which cannot be obtained by the multi-passprinting system, the use of the single-passing system which allowshigh-speed printing is desired.

The single-passing system often uses fixed ink-jet heads, and thusrequires head cleaning. Typically, head cleaning is carried out once perevery several hours, and the frequency is low. Therefore, thesingle-passing system requires the designing of ink which will not causedeflection or ejection failure of ink. In particular, when titaniumoxide is used as the white pigment of a white ink, specific gravity ofthe pigment in the ink is high. Therefore, when the apparatus is ceasedfor a while, the pigment tends to sediment in the nozzles. Furthermore,pigment sedimentation tends to cause problems such as ink ejectionfailure (nozzle void), so that improvement is desired.

Patent Literatures 1 to 3 disclose the methods for preventingsedimentation of titanium oxide through the surface treatment oftitanium oxide used as the pigment, and/or the use of a specific pigmentdispersant. Prevention of sedimentation of titanium oxide likelyimproves ejection stability. However, by any of these disclosed methods,it is difficult to achieve sufficient ejection stability over a longtime which allows favorable use of inks on the single-passing system.

Patent Literature 4 discloses a method of improving pigmentsedimentation by using hollow particles in place of titanium oxide asthe white pigment. The hollow particles have a lower specific gravitythan titanium oxide, so that the hollow particles resist sedimentationeven in a low viscosity ink. However, the hollow particles tend to havelower hiding property in comparison with titanium oxide. Therefore, inorder to achieve a sufficient printing concentration by printing on thesingle-passing system, for example, the printing concentration must beimproved by using multiple heads, which may cause a problem of capsizingof the apparatus.

CITATION LIST Patent Literatures

Patent Literature 1: Japanese Patent No. 4799987

Patent Literature 2: Japanese Patent No. 4807816

Patent Literature 3: Japanese Patent No. 4979177

Patent Literature 4: Japanese Patent No. 4902216

SUMMARY OF INVENTION Technical Problem

In view of the above-described circumstances, the present invention isintended to provide an ink-jet white ink of active energy ray-curabletype, wherein the white ink has high printing stability and good hidingproperty, and is particularly suitable for printing on the single-passink-jet printing system.

Solution to Problem

One embodiment of the present invention relates to an ink-jet white inkof active energy ray-curable type, comprising titanium oxide, a pigmentdispersant, and a polymerizable compound, wherein the content of thetitanium oxide is from 15 to 30% by weight with reference to the totalweight of the ink, and wherein the weight average particle diameter ofthe titanium oxide is from 180 to 250 nm.

In the white ink, the titanium oxide preferably includes titanium oxidetreated with a surface-treating agent. The titanium oxide preferably hashydroxyl groups on treated surface thereof. In addition, in the whiteink, the titanium oxide is preferably the titanium oxide whose surfaceis treated with silica.

In the white ink, the pigment dispersant preferably includes a basicdispersant.

In the white ink, the polymerizable compound preferably includes atleast one of the polymerizable compounds represented by the followinggeneral formulae (1) and (2):

(R1 and R2 are each independently a hydrogen atom or a methyl group, R3is a hydrogen atom or an alkyl group with 1 to 10 carbon atoms, and nrepresents an integer of 1 to 10.)

(R1 is a hydrogen atom or a methyl group, R3 is a hydrogen atom or analkyl group with 1 to 10 carbon atoms, and m represents an integer of 1to 10.).

It is preferable that the white ink further includes a polymerizationinitiator and that the polymerization initiator includes at least oneselected from the group consisting of2,4,6-trimethylbenzoyldiphenylphosphine oxide,bis(2,4,6-trimethylbenzoyl)phenylphosphine oxide, and4-benzoyl-4′-methyldiphenyl sulfide.

It is preferable that the white ink further includes an organic solventand that the content of the organic solvent is from 0.01 to 5% by weightwith reference to the total weight of the ink.

It is preferable that the white ink is used for printing on thesingle-pass ink-jet printing system.

Advantageous Effects of Invention

The present invention provides an ink-jet white ink of active energyray-curable type, the white ink having high printing stability and goodhiding property. The white ink is suitable for printing on thesingle-pass ink-jet printing system.

Disclosure of the present invention relates to the subject of JapanesePatent Application No. 2013-092483 filed on Apr. 25, 2013, and thedisclosure of which is incorporated by reference herein.

DESCRIPTION OF EMBODIMENTS

Details about the present invention are described below based onpreferred embodiments.

<White Ink for Ink-Jet Printing>

One embodiment of the present invention relates to an ink-jet white inkof active energy ray-curable type (hereinafter referred to as “whiteink”). The white ink is intended to be used under the printing systemwherein ink is jetted and landed on a base material to be printed by theink-jet printing system, and then the ink coating film is cured byirradiation with active energy ray, thereby an image and characters areformed. The term “active energy rays” herein means the energy beams in abroad sense which can provide energy necessary for the activation ofchemical reaction. Not particularly limited, but in one embodiment ofthe present invention, the active energy rays are preferably lightenergy containing ultraviolet rays. The white ink contains titaniumoxide, a pigment dispersing resin, and a polymerizable compound, thecontent of titanium oxide is from 15 to 30% by weight with reference tothe total weight of the white ink, and the titanium oxide has a weightaverage particle diameter of 180 to 250 nm. The components of the whiteink are more specifically described below.

(Pigment)

The white ink of the present invention includes titanium oxide having aweight average particle diameter of 180 to 250 nm as the white pigment.The titanium oxide may be untreated or surface-treated. In the whiteink, if the weight average particle diameter of the titanium oxide istoo small, sufficient hiding property cannot be achieved. On the otherhand, if the weight average particle diameter of the titanium oxide istoo large, sedimentation of the titanium oxide is hastened. Therefore,titanium oxide included in the ink in the ink-jet head sediments in ashort time, and decap property tends to deteriorate. Regarding this, inthe present invention, the titanium oxide has a weight average particlediameter of 180 to 250 nm, so that decap property during printing isimproved, and printing is successfully carried out over a long time. Inone embodiment, the weight average particle diameter of the titaniumoxide is preferably from 200 to 250 nm, and more preferably from 220 to250 nm.

The term “weight average particle diameter” used herein means not theprimary particle diameter of titanium oxide, but the particle diameterof titanium oxide dispersed in the ink, more specifically the secondaryparticle diameter. The secondary particle diameter can be confirmed by,for example, a particle diameter distribution analyzer of dynamic lightscattering type.

In the present invention, the primary particle diameter of the titaniumoxide used as the ink material is not particularly limited. However,when titanium oxide having a primary particle diameter larger than therange of the secondary particle diameter defined in the white ink of thepresent invention is used, the primary particles must be micronized.Such micronization causes destruction of titanium oxide, or peeling ofthe treating agent from the surface of titanium oxide, and tends tocause deterioration of dispersion stability or inclusion of coarseparticles. Accordingly, in one embodiment, it is preferred that titaniumoxide having a primary particle diameter of 150 to 250 nm is used, andthe titanium oxide is subjected to dispersion treatment, therebyadjusting them to the range of the secondary particle diameter definedin present invention. More preferably, titanium oxide having a primaryparticle diameter of 180 to 240 nm is used, and even most preferably,titanium oxide having a primary particle diameter of 200 to 230 nm isused.

The content of titanium oxide in the white ink is preferably from 15 to30% by weight with reference to the total weight of the white ink. Ifthe content of titanium oxide is too low, obtaining sufficient hidingproperty is hard. On the other hand, if the content is too high, storagestability tends to deteriorate. Regarding this, the content of titaniumoxide in the white ink of the present invention is adjusted within theabove-described range, so that high hiding property is achieved even ifthe thickness of the printed material is small, and high storagestability of the ink is maintained. The content of titanium oxide ispreferably from 17 to 25% by weight, and more preferably from 19 to 23%by weight with reference to the total weight of the white ink.

The titanium oxide used in the present invention may be of anatase orrutile type. Titanium oxide of rutile type is preferred, because itmakes it easy to improve the hiding property of the printed material. Inaddition, the titanium oxide to be used may be prepared by any methodsuch as chlorine method or sulfuric acid method. The titanium oxideprepared by chlorine method is preferred for improving whiteness of theprinted material.

In one preferred embodiment of the present invention, the use ofsurface-treated titanium oxide is preferred. The surface treatment maybe carried out using a surface-treating agent, and the surface-treatingagent may be inorganic or organic. The treatment with thesurface-treating agent makes it easy to reduce catalytic activity andcontrol hydrophilicity on the surface of titanium oxide. In addition,weather resistance and dispersion stability of the ink are readilyimproved. Examples of the inorganic surface-treating agent includealuminum compounds including aluminum hydroxide and alumina, silica,zirconia, tin, antimony, and titanium. Examples of the organicsurface-treating agent include polyalcohols, alkanolamines,organosilicon compounds, organic phosphate compounds, and higher fattyacids.

As an embodiment of the white pigment, titanium oxide whose surface istreated with aluminum and/or silica is preferred. Of these embodiments,titanium oxide whose surface is treated with aluminum and silica is morepreferred. As is evident from the below-described example, inparticular, the embodiment including the combination of a basicdispersant and titanium oxide whose surface is treated with silica tendsto more highly improve dispersion stability and pigment sedimentationproperties than other embodiments. Therefore, according to theembodiment of the white ink including the titanium oxide whose surfaceis treated with at least silica, decap property during printing arereadily improved.

Commonly, titanium oxide whose surface is treated with silica is knownto have acidic hydroxyl groups on particle surface thereof. Therefore,the combination of the titanium oxide and a basic dispersant formsintermolecular interaction such as a hydrogen bond between them.Although not bound by any theory, such intermolecular interaction likelyimproves adsorption between the titanium oxide and basic dispersant, andmakes it easy to control the sedimentation of titanium oxide. Inaddition, for the same reason, among the titanium oxide treated with theabove-described organic surface-treating agent, the titanium oxidetreated with a polyalcohol, alkanolamine, or organosilicon compound isparticularly preferred, because it has neutral hydroxyl groups onparticle surface thereof.

From these viewpoints, in one embodiment of the present invention, thetitanium oxide is more preferably treated with a surface-treating agent,and the treated surface has hydroxyl groups. The hydroxyl groups areclassified into acidic, neutral, and basic according to thesurface-treating agent used. In the present invention, from theviewpoint of interaction with the basic dispersant, the hydroxyl groupsare preferably acidic and neutral, and more preferably acidic.

In one embodiment of the present invention, the titanium oxide having aweight average particle diameter of 180 to 250 nm, or a primary particlediameter of 150 to 250 nm is preferably used. Such titanium oxide iscommercially available. Examples include TIPAQUE CR-60, CR-60-2, CR-63,CR-67, PF-690, PF-691, PF-726, and PF-728 manufactured by IshiharaSangyo Kaisha, Ltd; KRONOS 2064, 2160, 2190, 2220, 2300, and 2310manufactured by KRONOS Worldwide Inc. Among them, TIPAQUE CR-63, CR-80,PF-690, PF-691, PF-726, PF-728, KRONOS 2220, and 2310 are preferred, allof which are titanium oxide having hydroxyl groups on the treatedsurface.

(Pigment Dispersant)

The white ink of the present invention preferably contains a pigmentdispersant. The use of an appropriate pigment dispersant improvespigment dispersibility and ink storage stability, and makes it easy toprevent sedimentation of the pigment. In one embodiment of the presentinvention, a basic dispersant is preferably used from the viewpoints ofaffinity with the ink constituents, and storage stability. In addition,from the viewpoint of affinity with the ink constituents, it isparticularly preferred that a pigment dispersant of resin type be used.Specific examples of the pigment dispersant of resin type includepolymeric dispersants having a main skeleton such as polyurethaneskeleton, polyacryl skeleton, polyester skeleton, polyamide skeleton,polyimide skeleton, and polyurea skeleton. From the viewpoint of storagestability of the ink, a polymeric dispersant having at least oneselected from a polyurethane skeleton, a polyacryl skeleton, and apolyester skeleton is preferred. The structure of the polymericdispersant is not particularly limited. For example, the structure maybe a random structure, a block structure, a comb structure, or a starstructure. From the viewpoint of storage stability of the ink, apolymeric dispersant having a block structure or a comb structure ispreferred.

The weight average molecular weight of the pigment dispersant used inthe present invention is preferably 1000 or more and 70000 or less. Themolecular weight is more preferably 2000 or more and 60000 or less, andparticularly preferably 3000 or more and 50000 or less. When themolecular weight is adjusted within this range, pigment dispersibilityis improved while keeping the ink viscosity low, and further pigmentsedimentation is readily prevented.

Specific examples of the pigment dispersant used in the presentinvention are listed below:

wetting dispersants manufactured by BYK-Chemie, including DISPER BYK101, 102, 103, 106, 108, 109, 110, 111, 112, 116, 130, 140, 142, 145,161, 162, 163, 164, 166, 167, 168, 170, 171, 174, 180, 182, 183, 184,185, 2000, 2001, 2020, 2050, 2070, 2096, and 2150; BYK JET 9130, 9131,9132, 9133, and 9150; and LP-N-22252;

EFKA series manufactured by Ciba Specialty Chemicals, including 4008,4009, 4010, 4015, 4020, 4046, 4047, 4050, 4055, 4060, 4080, 4300, 4330,4340, 4400, 4401, 4402, 4403, 4406, 4800, 5010, 5044, 5054, 5055, 5063,5064, 5065, 5066, 5070, and 5244;

Solsperse series manufactured by The Lubrizol Corporation, including3000, 11200, 13240, 13650, 13940, 16000, 17000, 18000, 20000, 21000,24000SC, 24000GR, 26000, 28000, 31845, 32000, 32500, 32550, 32600,33000, 34750, 35100, 35200, 36000, 36600, 37500, 38500, 39000, 41000,53000, 53095, 54000, 55000, 56000, 71000, 76400, 76500, J100, and J200;

DISPARLON series manufactured by Kusumoto Chemicals, Ltd., including1210, 1220, 1831, 1850, 1860, 2100, 2150, 2200, 7004, KS-260, KS-273N,KS-860, KS-873N, PW-36, DN-900, DA-234, DA-325, DA-375, DA-550, DA-1200,DA-1401, and DA-7301;

AJISPER series manufactured by Ajinomoto Fine-Techno Co., Inc.,including PB-711, PB-821, PB-822, PB-824, PB-827, PB-711,PN-411, andPA-111.

In one embodiment of the present invention, DISPER-BYK 162, 167, 168,and LP-N-22252; Solsperse 24000SC, 24000GR, 32000, 33000, 35000, 39000,76400, 76500, J100, and J200; and AJISPER PB-821, 822, 824, and 827 arepreferred. All of them are dispersants of basic resin type.

The content of the pigment dispersant is not particularly limited, andmay be freely adjusted as long as desired stability is achieved. In oneembodiment of the present invention, it is preferred that titanium oxidetreated with a surface-treating agent, and having hydroxyl groups on thetreated surface be used as a white pigment. In this embodiment, in orderto sufficiently achieve the above-described effect of intermolecularinteraction using a basic resin dispersant as the pigment dispersant, itis important to appropriately adjust the mixing ratio between thetitanium oxide and pigment dispersant. Accordingly, in one embodiment ofthe present invention, from the viewpoints of pigment dispersibility,storage stability of the ink, and decap property, the proportion of theactive ingredients of the pigment dispersant is preferably from 2 to 10parts by weight with reference to 100 parts by weight of titanium oxide.When the mixing ratio is adjusted within this range, pigmentdispersibility is good, pigment sedimentation is inhibited even if thestanding time after printing is prolonged, and nozzle void hardlyoccurs. On the other hand, if the content of the pigment dispersant inthe ink is too small, it is difficult to maintain dispersion stabilityof the pigment, and pigment flocculation tends to occur. In addition, ifthe content of the pigment dispersant is too high, ink flowabilitydeteriorates, and ejection property may be influenced. For this reason,in one embodiment of the present invention, the content of the pigmentdispersant is preferably from 3 to 8 parts by weight, and morepreferably from 4 to 7 parts by weight with reference to 100 parts byweight of titanium oxide.

(Polymerizable Compound)

The white ink of the present invention may use a polymerizable compoundwell known to the technical field. The polymerizable compound may be anycompound as long as it has at least one polymerizable functional groupin a molecule, and such compounds may be used in combination. Althoughnot specifically limited, in one embodiment, the ink preferably includesat least one of the compounds represented by the general formulae (1)and (2). These specific compounds have a low viscosity, and thus candecrease the ink viscosity. Therefore, the use of the compound makesreadily improves the ink ejection performance. In addition, reduction ofthe ink viscosity allows the increase of the content of the pigment andpolymerization initiator. Accordingly, the use of the compound allowedthe improvement of the hiding property and curability of the printedmaterial.

In the formula, R1 and R2 are each independently a hydrogen atom or amethyl group, R3 is a hydrogen atom or an alkyl group with 1 to 10carbon atoms, and n represents an integer of 1 to 10. R3 is preferably ahydrogen atom, and n is preferably 1 or 2. If n is greater than 10, thepolymerizable compound has a high viscosity, which may makes itdifficult to obtain an ink with a low viscosity.

In the formula, R1 is a hydrogen atom or a methyl group, R3 is ahydrogen atom or an alkyl group with 1 to 10 carbon atoms, and mrepresent an integer of 1 to 10. R3 is preferably a hydrogen atom. If mis greater than 10, the polymerizable compound has a high viscosity,which may makes it difficult to obtain an ink with a low viscosity.

Specific examples of the polymerizable compounds represented by thegeneral formulae (1) and (2) include vinyloxyethyl (meth) acrylate,vinyloxydiethylene glycol (meth)acrylate, vinyloxytriethylene glycol(meth) acrylate, vinyloxypropylene glycol (meth) acrylate,vinyloxydipropylene glycol (meth) acrylate, vinyloxytripropylene glycol(meth) acrylate, vinyloxymethyl(meth)acrylate, vinyloxybutyl (meth)acrylate, vinyloxyhexyl (meth) acrylate, vinyloxyoctyl (meth) acrylate,and vinyloxydecyl (meth)acrylate. Among them, vinyloxyethyl (meth)acrylate, vinyloxydiethylene Glycol (meth) acrylate, vinyloxypropyleneglycol (meth) acrylate, and vinyloxybutyl (meth)acrylate are preferred.In one embodiment, vinyloxyethyl (meth)acrylate and vinyloxypropyleneglycol (meth)acrylate are more preferred, and vinyloxyethyl(meth)acrylate is most preferred. According to another embodiment,vinyloxydiethylene glycol (meth)acrylate is preferred, andvinyloxydiethylene glycol acrylate (VEEA) is most preferred. VEEA is acompound represented by the formula (1), wherein R1 to R3 are hydrogenatoms, and n=2.

The content of the specific compounds represented by the generalformulae (1) and (2) is preferably from 10 to 50% by weight, morepreferably from 15 to 45% by weight, and most preferably from 20 to 40%by weight with reference to the total weight of the ink. When thecontent of the specific compounds is adjusted within this range, the inkviscosity ink is readily decreased while a storage stability of the inkis maintained. Furthermore, as a result of this, ejection performance ofthe ink is readily improved. When the content of the specific compoundin the ink is 10% by weight or more, ink viscosity is readily decreased,and as a result of this, ejection performance of the ink is improved. Onthe other hand, when the content is 50% by weight or less, the pigmentdispersant in the ink dissolves, and as a result of this, the problem ofdeficiency of the amount of the pigment dispersant to titanium oxide isavoided. Accordingly, marked storage stability of the ink is readilyachieved.

According to another embodiment, the polymerizable compound may be acombination of a monofunctional monomer and a polyfunctional monomer.The monofunctional monomer means a compound having only onepolymerizable functional group in a molecule. The polyfunctional monomermeans a compound having two or more polymerizable functional groups in amolecule. However, in the present description, the specific compoundsrepresented by the general formulae (1) and (2) are not included.According to vet another embodiment, the polymerizable compound may be acombination of a monofunctional monomer, a polyfunctional monomer, andspecific compounds represented by the general formulae (1) and (2).

Specific examples of the monofunctional monomer include benzyl(meth)acrylate, (ethoxylated (or propoxylated))-2-phenoxyethyl (meth)acrylate, dicyclopentenyl(oxyethyl) (meth) acrylate, 2-methoxyethyl(meth)acrylate, methoxy triethylene glycol (meth)acrylate, 2-ethoxyethyl(meth) acrylate, ethoxy ethoxyethyl (meth)acrylate, methoxy dipropyleneglycol (meth)acrylate, β-carboxylethyl (meth) acrylate,trimethylolpropane formal (meth) acrylate, isoamyl (meth) acrylate,cyclohexyl (meth) acrylate, tetrahydrofurfuryl (meth) acrylate,isoboronyl (meth) acrylate, dicyclopentanyl (meth) acrylate, isooctyl(meth)acrylate, lauryl (meth)acrylate, 2-hydroxy-3-phenoxy propyl(meth)acrylate, 1,4-cyclohexane dimethanol (meth)acrylate,2-hydroxyethyl (meth)acrylate, 2-hydroxypropyl (meth) acrylate,4-hydroxybutyl (meth) acrylate, acryloyl morpholine, N-vinylcaprolactam, N-vinylpyrrolidone, N-vinylformamide, and N-acryloyloxyethyl hexahydrophthalimide.

Specific examples of the polyfunctional monomer includedimethylol-tricyclodecane di(meth)acrylate, (ethoxylated (orpropoxylated)) bisphenol A di(meth)acrylate, cyclohexane dimethanoldi(meth)acrylate, (poly)ethylene glycol di(meth)acrylate, (ethoxylated(or propoxylated)) 1,6-hexanediol di(meth)acrylate, 1,9-nonanedioldi(meth)acrylate, 1,10-decanediol di(meth)acrylate, (ethoxylated (orpropoxylated)) neopentyl glycol di(meth)acrylate, hydroxypivalic acidneopentyl glycol di(meth)acrylate, dipropylene glycol di(meth)acrylate,(neopentyl glycol-modified) trimethylolpropane di(or tri)(meth)acrylate,tripropylene glycol di(meth)acrylate, dicyclopentanyl di(meth)acrylate,pentaerythritol tri(or tetra) (meth) acrylate, ditrimethylolpropanetri(or tetra)(meth)acrylate, tetramethylol methane tri(ortetra)(meth)acrylate, and dipentaerythritol hexa(meth)acrylate.

In one embodiment, the monofunctional monomer is preferably at least oneselected from the group consisting of phenoxyethyl acrylate, ethoxyethoxyethyl acrylate, isoboronyl acrylate, isooctyl acrylate, laurylacrylate, and N-vinyl caprolactam. In addition, the polyfunctionalmonomer is preferably at least one selected from the group consisting of(ethoxylated (or propoxylated)) neopentyl glycol diacrylate,1,6-hexanediol diacrylate, 1,9-nonanediol diacrylate, 1,10-decanedioldiacrylate, dipropylene glycol diacrylate, and tripropylene glycoldiacrylate. Furthermore, in terms of ink curability and odor, the use ofat least one selected from the group consisting of lauryl acrylate,dipropylene glycol diacrylate, N-vinyl caprolactam, 1,9-nonanedioldiacrylate, and 1,10-decanediol diacrylate is more preferred.

In the present invention, the whole composition of the polymerizablecompound is not particularly limited. However, in the whole composition,if the proportion of the monofunctional monomer is too high, inkcurability may decrease. Therefore, in one embodiment, with reference tothe total weight of the polymerizable compound, the total content of thepolyfunctional monomer and/or the specific compounds represented by thegeneral formulae (1) and (2) is preferably 70% by weight or more. Thetotal content is more preferably 80% by weight or more, and even morepreferably 90% by weight or more. In one embodiment, with reference tothe total weight of the polymerizable compound, the specific compoundsrepresented by the general formulae (1) and (2) is more preferably 70%by weight or more. When the content of the specific compound is 70% byweight or more, reduction of the ink viscosity, and improvement ofejection stability are readily achieved. In either embodiment, thecontent of the polymerizable compound in the ink is preferably from 50to 80% by weight, more preferably from 55 to 75% by weight, and mostpreferably from 60 to 70% by weight. When the content of thepolymerizable compound is adjusted within this range, the ink viscosityis maintained at an appropriate value, and ejection stability is readilyimproved.

The white ink of the present invention may further include, in additionto the above-described constituent, other components known to thewell-known in the art. For example, in one embodiment, an oligomerand/or a prepolymer may be used. Specific examples include, but notlimited to, the followings.

“Ebecryl 230, 244, 245, 270, 280/151B, 284, 285, 4830, 4835, 4858, 4883,8402, 8803, 8800, 254, 264, 265, 294/35HD, 1259, 1264, 4866, 9260, 8210,1290, 1290K, 5129, 2000, 2001, 2002, 2100, KRM7222, KRM7735, 4842, 210,215, 4827, 4849, 6700, 6700-20T, 204, 205, 6602, 220, 4450, 770, IRR567,81, 84, 83, 80, 657, 800, 805, 808, 810, 812, 1657, 1810, IRR302, 450,670, 830, 835, 870, 1830, 1870, 2870, IRR267, 813, IRR483, 811, 436,438, 446, 505, 524, 525, 554W, 584, 586, 745, 767, 1701, 1755, 740/40TP,600, 601, 604, 605, 607, 608, 609, 600/25TO, 616, 645, 648, 860, 1606,1608, 1629, 1940, 2958, 2959, 3200, 3201, 3404, 3411, 3412, 3415, 3500,3502, 3600, 3603, 3604, 3605, 3608, 3700, 3700-20H, 3700-20T, 3700-25R,3701, 3701-20T, 3703, 3702, RDX63182, 6040, IRR419” manufactured byDaicel-UCB Company, Ltd.;

“CN104, CN120, CN124, CN136, CN151, CN2270, CN2271E, CN435, CN454,CN970, CN971, CN972, CN9782, CN981, CN9893, and CN991” manufactured bySartomer Company, Inc.;

“Laromer EA81, LR8713, LR8765, LR8986, PE56F, PE44F, LR8800, PE46T,LR8907, P043F, P077F, PE55F, LR8967, LR8981, LR8982, LR8992, LR9004,LR8956, LR8985, LR8987, UP35D, UA19T, LR9005, P083F, P033F, P084F,P094F, LR8863, LR8869, LR8889, LR8997, LR8996, LR9013, LR9019, P09026V,and PE9027V” manufactured by BASF;

“Photomer 3005, 3015, 3016, 3072, 3982, 3215, 5010, 5429, 5430, 5432,5662, 5806, 5930, 6008, 6010, 6019, 6184, 6210, 6217, 6230, 6891, 6892,6893-20R, 6363, 6572, and 3660” manufactured by Cognis Corporation;

“Art-Resin UN-9000HP, 9000PEP, 9200A, 7600, 5200, 1003, 1255, 3320HA,3320HB, 3320HC, 3320HS, 901T, 1200TPK, 606OPTM, and 6060P” manufacturedby Negami Chemical Industrial Co., Ltd.;

“Shiko UV-6630B, 7000B, 7510B, 7461TE, 3000B, 3200B, 3210EA, 3310B,3500BA, 3520TL, 3700B, 6100B, 6640B, 1400B, 1700B, 6300B, 7550B, 7605B,7610B, 7620EA, 7630B, 7640B, 2000B, 2010B, 2250EA, and 2750B”manufactured by Nippon Synthetic Chemical Industry Co., Ltd.;

“KAYARAD R-280, R-146, R131, R-205, EX2320, R190, R130, R-300, C-0011,TCR-1234, ZFR-1122, UX-2201, UX-2301, UX3204, UX-3301, UX-4101, UX-6101,UX-7101, MAX-5101, MAX-5100, MAX-3510, and UX-4101” manufactured byNippon Kayaku Co., Ltd.; and the like.

When any of these oligomer and/or prepolymer component is used, thecontent of the component is preferably less than 10% by weight, and morepreferably less than 5% by weight, with reference to the total weight ofthe ink.

(Polymerization Initiator)

In one embodiment of the present invention, the active energy raysapplied to the white ink are preferably light energy containingultraviolet ray. In this embodiment, it is preferable that theabove-described constituents of the white ink further include apolymerization initiator. The polymerization initiator is notparticularly limited, and may be appropriately selected from thecompounds known in the technical field, in consideration of the curingrate and physical properties of the cured coating film. In oneembodiment, a molecule-cleaving or hydrogen-withdrawing polymerizationinitiator is preferred. Specific examples of the polymerizationinitiator include benzoin isobutyl ether, 2,4-diethylthioxanthone,2-isopropylthioxanthone, 2,4,6-trimethylbenzoyl diphenyl phosphineoxide, 2-benzyl-2-dimethylamino-1-(4-morpholinophenyl)-butane-1-one,bis(2,4,6-dimethoxybenzoyl)-2,4,4-trimethylpentylphosphine oxide,bis(2,4,6-trimethylbenzoyl) phenylphosphine oxide,2-methyl-1-(4-methylthio phenyl)-2-morpholinopropane-1-one,oligo(2-hydroxy-2-methyl-1-(4-(1-methylvinyl)phenyl)propanone),4-benzoyl-4′-methyl-diphenyl sulfide, 1,2-octanedione, and1-(4-(phenylthio)-2,2-(O-benzoyloxime)). These polymerization initiatorsare preferred because their radical formation reaction will not beinhibited by light absorption of a pigment and a polymerizable compound,and their high radical generation efficiency improves ink curability.The content of the polymerization initiator is preferably from 3 to 20%by weight, and more preferably from 5 to 15% by weight with reference tothe total weight of the ink.

Other specific examples of the polymerization initiator includemolecule-cleaving polymerization initiators such as1-hydroxycyclohexylphenyl ketone, benzoin ethyl ether, benzyldimethylketal, 2-hydroxy-2-methyl-1-phenylpropane-1-one, and1-(4-isopropyl phenyl)-2-hydroxy-2-methylpropane-1-one. In addition,they include hydrogen-withdrawing polymerization initiators such asbenzophenone, 4-phenylbenzophenone, and isophthalphenone.

Among the above-listed polymerization initiators, at least one selectedfrom the group consisting of 2,4,6-trimethylbenzoyldiphenylphosphineoxide, bis(2,4,6-trimethylbenzoyl)phenylphosphine oxide, and4-benzoyl-4′-methyldiphenyl sulfide is preferred. These polymerizationinitiators are preferred because they have high curability, and will notyellow the cured ink film. The ink of the present invention includesone, preferably two of the above-listed polymerization initiators. Inparticular, the use of the combination of2,4,6-trimethylbenzoyldiphenylphosphine oxide and4-benzoyl-4′-methyldiphenyl sulfide is preferred. In a more preferredembodiment, the mixing ratio of 2,4,6-trimethylbenzoyldiphenylphosphineoxide and 4-benzoyl-4′-methyldiphenyl sulfide is from 2:1 to 5:1. Inthis embodiment, the total content of the polymerization initiator ispreferably from 10 to 20% by weight with reference to the total weightof the ink.

(Sensitizer)

In one embodiment of the white ink of the present invention, thepolymerization initiator may be combined with a sensitizer. When thepolymerization initiator and a sensitizer are used in combination, thesensitizer is preferably an amine which will not cause addition reactionwith the polymerizable compound. Examples of the sensitizer useful inthe present invention include trimethylamine, methyldimethanol amine,triethanolamine, p-diethylaminoacetophenone,ethyl-4-(dimethylamino)-benzoate, Isoamyl p-(dimethylamino)-benzoate,N,N-dimethylbenzylamine, and 4,4T-bis(diethylamino)benzophenone. Thesesensitizers may yellow the cured ink film, and thus are preferably usedin an amount which will not cause yellowing.

(Organic Solvent)

In one embodiment of the white ink of the present invention, the use ofan organic solvent is preferred. The use of an organic solvent makes iteasy to decrease the ink viscosity, and allows the improvement ofejection property. Examples of the organic solvent include glycolmonoacetate compounds such as ethylene glycol monobutyl ether acetate,diethylene glycol monoethyl ether acetate, diethylene glycol monobutylether acetate; glycol ether compounds such as diethyleneglycol dimethylether, diethylene glycol methyl ethyl ether, diethylene glycol diethylether, and tetraethylene glycol dimethyl ether; and lactate compoundssuch as ethyl lactate, and butyl lactate. Among them, ethylene glycolmonobutyl ether acetate, diethylene glycol monobutyl ether acetate,diethylene glycol methyl ethyl ether, diethylene glycol diethyl etherare preferred, and diethylene glycol methyl ethyl ether, diethyleneglycol diethyl ether are more preferred.

The content of the organic solvent is preferably from 0.01 to 5% byweight with reference to the total weight of the ink. When the contentis 0.01% by weight or more, ejection property tends to be good. Inaddition, when the content is 5% by weight or less, ejection is stablycarried out without deterioration of the decap property caused by dryingof the organic solvent. The content is more preferably from 0.1 to 3% byweight, and even more preferably from 0.5 to 2% by weight.

(Surface Conditioner)

In one embodiment of the white ink of the present invention, the use ofa surface conditioner is preferred, thereby improving spreadability onthe base material. The surface conditioner may be any compound known inthe technical field, and may be appropriately selected from acrylic,silicon, and fluorine compounds. Specific examples of the surfaceconditioner include, but not limited to, BYK-350, 352, 354, 355, 358N,361N, 381N, 381, 392, BYK-300, 302, 306, 307, 310, 315, 320, 322, 323,325, 330, 331, 333, 337, 340, 344, 370, 375, 377, 355, 356, 357, 390,UV3500, UV3510, and UV3570 manufactured by BYK-Chemie. Other examplesinclude Tegoglide-100, 110, 130, 420, 432, 435, 440, 450GZ400,Tegorad-2100, 2200, 2250, 2500, and 2700 manufactured by Evonik DegussaGmbH. These surface conditioners may be used singly or in combination oftwo or more thereof according to the intended use.

The content of the surface conditioner is preferably from 0.01 to 5% byweight with reference to the total weight of the ink. When the contentis 0.01% by weight or more, spreadability tends to be good. On the otherhand, when the content is 5% by weight or less, storage stability of theink tend to be better. The content is more preferably from 0.05 to 3% byweight.

(Stabilizer)

In one embodiment of the white ink of the present invention, the use ofa stabilizer is preferred. The use of a stabilizer improves viscositystability of the ink over time, and on-machine viscosity stability inthe recording apparatus. The stabilizer useful in the present inventionis not particularly limited. In one preferred embodiment, at least oneselected from the group consisting of hindered phenol compounds,phenothiazine compounds, hindered amine compounds, and phosphoruscompounds may be used. Specific examples of the stabilizer are listedbelow.

Hindered phenol compounds: “IRGANOX 1010, 1010FF, 1035, 1035FF, 1076,1076FD, 1076DWJ, 1098, 1135, 1330, 245, 245FF, 245DWJ, 259, 3114, 565,565DD, and 295” manufactured by BASF; “BHT SWANOX”, “NONFLEX Alba, MBP,EBP, CBP, BB”, “TBH” manufactured by Seiko Chemical Co., Ltd., “AO-20,30, 50, 50F, 70, 80, and 330” manufactured by ADEKA Corporation; “H-BHT”manufactured by Honshu Chemical Industry Co., Ltd., and “Yoshinox BB,425, and 930” manufactured by API Corporation.

Phenothiazine compounds: “Phenothiazine” manufactured by Seiko ChemicalCo., Ltd.; “Phenothiazine”, “2-methoxyphenothiazine”, and“2-cyanophenothiazine” manufactured by Sakai Chemical Industry Co., Ltd.

Hindered amine compounds: “IRGANOX 5067” “TINUVIN 144, 765, 770DF, and622LD” manufactured by BASF; “NONFLEX H, F, OD-3, DCD, and LAS-P”,“STEARER STAR”, “diphenylamine”, “4-aminodiphenylamine”, and“4-oxydiphenylamine” manufactured by Seiko Chemical Co., Ltd.; “HOTEMPO”manufactured by Evonik Degussa GmbH; “Fancryl 711MM and 712HM”manufactured by Hitachi Chemical Co., Ltd.

Phosphorus compounds: “triphenylphosphine”, “IRGAFOS 168 and 168FF”manufactured by BASF; and “NONFLEX TNP” manufactured by Seiko ChemicalCo., Ltd.

Other compounds: “IRGASTAB UV-10 and 22” manufactured by BASF;“Hydroquinone”, “Metokinon”, “Torukinon”, “NH”, “PBQ”, “TBQ”, and“2,5-diphenyl-p-benzoquinone” manufactured by Seiko Chemical Co., Ltd.;“Q-1300 and 1301” manufactured by Wako Pure Chemical Industries, Ltd.;and “GENORAD 16, 18, and 20” manufactured by RAHN AG.

Among the above-listed compounds, the following compounds are preferredfrom the viewpoints of solubility in the ink and color of thestabilizer.

Hindered phenol compounds: “BHT Swanox”, “NONFLEX Alba” manufactured bySeiko Chemical Co., Ltd.; “H-BHT” manufactured by Honshu ChemicalIndustry Co., Ltd.

Phenothiazine compounds: “Phenothiazine” manufactured by Seiko ChemicalCo., Ltd.; “Phenothiazine” manufactured by Sakai Chemical Industry Co.,Ltd.

Hindered amine compounds: “HO-TEMPO” manufactured by Evonik DegussaGmbH.

Phosphorus compounds: “Triphenyl phosphine” manufactured by BASF.

The content of the stabilizer is preferably from 0.01 to 5% by weightwith reference to the total weight of the ink. If the content is belowthis range, ink stability may be poor. On the other hand, if the contentis above this range, ink curing may be hindered.

The white ink of the present invention may be prepared by a method wellknown in the technical field. In one embodiment, the white ink may beprepared as follows: titanium oxide, a pigment dispersing resin, and apolymerizable monomer are uniformly mixed and dispersed to make apigment dispersion, and then other constituents such as a polymerizablemonomer and a polymerization initiator are added to the dispersion, anduniformly mixed. This dispersion treatment may be carried out by, forexample, a wet bead mill using a pulverization medium. The pulverizationmedium is preferably zirconia beads. The diameter of the beads used asthe pulverization medium is preferably from 0.1 to 1 mm, and morepreferably from 0.3 to 0.5 mm. In addition, the polymerizable monomerused for dispersion treatment is preferably vinyloxydiethylene glycolacrylate or dipropylene Glycol diacrylate. In one embodiment, the inkthus obtained is preferably filtrated through a filter.

(Ink Properties)

The white ink of the present invention is intended to be used forprinting on the ink-jet printing system. Therefore, the ink preferablyhas a viscosity suitable for achieving stable ejection property at thetemperature during ink ejection. In one embodiment, the ink viscosity at25° C. is preferably adjusted to 5 to 50 mPa·s. When the ink viscosityis adjusted within this range, in particular, stable ejection propertyis achieved when an ordinary head having a frequency of 5 to 30 KHz.Ejection followability is readily improved by adjusting the viscosity to5 mPa·s or more. On the other hand, when the viscosity is adjusted to 50mPa·s or less, good ejection stability is readily obtained. If the inkviscosity is beyond the above-described range, ejection property tendsto deteriorate even if a viscosity-decreasing mechanism by heating isincorporated into the head, and ejection stability decreases, which mayresult in total failure of ejection. The ink viscosity can be adjustedby appropriately changing the mixing ratio of the constituents of theink. In one embodiment, the ink viscosity at 25° C. is more preferablyadjusted to 7 to 14.5 mPa·s, and particularly preferably adjusted to 8.5to 13 mPa·s. Stable ejection property is readily achieved by adjustingthe ink viscosity within this range when using an ordinary head having afrequency of 5 to 30 KHz, and also when using a head with a highfrequency of 10 to 50 KHz.

The white ink of the present invention is suitable to the ink-jetprinting system, particularly to the single-pass system (hereinafterreferred to as “single-pass ink-jet printing system”). The single-passink-jet printing system accomplishes printing on the base material to beprinted in one time, and is suitable for business printing required toachieve a high printing speed. Under this system, only one time ofprinting is carried out for one color, so that an ink with a highpigment concentration which can achieve a sufficient concentration inone time of printing is required. In addition, different from theshuttle-type multi-pass ink-jet printing system, a fixed ink-jet head isfrequently used under the single-pass ink-jet printing system. Under themulti-pass system, cleaning is periodically carried out in a shortperiod, but under the single system, the frequency of cleaning is aboutonce in every several hours, and the frequency is low. Therefore, inparticular, the single-pass ink-jet printing system requires an inkhaving high printing stability and superior decap property. Under suchcircumstances, prior art ink tends to cause the decrease of printingstability and decap property with the increase of the pigmentconcentration, so that it is difficult to satisfy the demand. On theother hand, the white ink of the present invention achieves high decapproperty, and prevents the occurrence of nozzle void even if cleaning isnot carried out for a long time. Therefore, the use of the white ink ofthe present invention allows satisfactory printing on the single-passink-jet printing system over a long time.

An alternative embodiment of the present invention relates to a printingmethod on the ink-jet printing system using the above-described whiteink. One embodiment of the printing method includes a step of ejectingthe droplets of a white ink from an ink-jet head on a printing basematerial, and forming a coating film of the white ink on the printingbase material, a step of irradiating the coating film with active energyrays to cure the coating film, thereby forming a printed surface. Theink-jet head may be a fixed head typically used under the single-passink-jet printing system. In addition, the amount of the ink solutiondroplets ejected from the ink-jet head may be 20 picoliter or less.According to the present invention, the printed material has high hidingproperty even if the ejection volume is 20 picoliter or less.

The printing base material used in the present invention is notparticularly limited. Examples of the printing base material includeplastic base materials such as polycarbonate, hard vinyl chloride, softvinyl chloride, polystyrene, styrofoam, PMMA, polypropylene,polyethylene, and PET. The printing base material may be a combinationor modified product of various plastic base materials. Other examplesinclude paper base materials such as fine paper, art paper, coatedpaper, and cast-coated paper, and metal base materials such as glass andstainless steel. In particular, the white ink of the present inventionhas high hiding property, so that is suitable to transparent basematerials and color base materials other than white base materials.

EXAMPLES

The present invention is more specifically described below based onexamples and comparative examples. Unless otherwise noted, “part” and“%” means “part by weight” and “% by weight”, respectively.

1. Preparation of Pigment Dispersion

(Pigment Dispersion A)

50 parts of TIPAQUE CR-60 (titanium oxide treated with aluminum, primaryparticle diameter 210 nm, manufactured by Ishihara Sangyo Kaisha, Ltd.)as a pigment, 2.5 parts of DISPER BYK-111 (acidic dispersant, acid value129 mmg KOH/g, manufactured by BYK-Chemie) as a pigment dispersant, and47.5 parts of dipropylene glycol diacrylate were placed in a disperser,and these components were mixed for preliminary dispersion.Subsequently, these components were subjected to main dispersion for 2hours using a 0.6-L DYNO mill filled with 1800 g of zirconia beadshaving a diameter of 1 mm, thereby obtaining a pigment dispersion A.

(Pigment Dispersion B1)

A pigment dispersion B1 was obtained in the same manner as in thepigment dispersion operation for the pigment dispersion A, except thatSOLSPERSE 32000 (basic dispersant, acid value 24, 8 mmg KOH/g, aminevalue 27, 1 mmg KOH/g, manufactured by The Lubrizol Corporation) wasused as a pigment dispersing resin.

(Pigment Dispersion B2)

A pigment dispersion B2 was obtained in the same manner as the pigmentdispersion B1, except that dipropylene glycol diacrylate in the pigmentdispersion B1, was replaced with vinyloxydiethylene glycol acrylate.

(Pigment Dispersion C1)

A pigment dispersion C1 was obtained in the same manner as in thepigment dispersion operation for the pigment dispersion A, except thatTIPAQUE PF-726 (titanium oxide treated with aluminum/silica, primaryparticle diameter 210 nm, manufactured by Ishihara Sangyo Kaisha, Ltd.)as the pigment, and SOLSPERSE 32000 (basic dispersant, manufactured byThe Lubrizol Corporation) was used as the pigment dispersing resin.

(Pigment Dispersion C2)

A pigment dispersion C2 was obtained in the same manner as in thepigment dispersion operation for the pigment dispersion C1, except thatdipropylene glycol diacrylate in the pigment dispersion C1, was replacedwith vinyloxydiethylene glycol acrylate.

(Pigment Dispersion D)

A pigment dispersion D was obtained in the same manner as in the pigmentdispersion operation for the pigment dispersion A, except that TIPAQUEA-220 (titanium oxide treated with aluminum, primary particle diameter160 nm, manufactured by Ishihara Sangyo Kaisha, Ltd.) as the pigment,and SOLSPERSE 32000 (basic dispersant, manufactured by The LubrizolCorporation) was used as the pigment dispersing resin.

(Pigment Dispersion E)

A pigment dispersion E was obtained in the same manner as in the pigmentdispersion operation for the pigment dispersion A, except that TIPAQUECR-58 (titanium oxide treated with aluminum, primary particle diameter280 nm, manufactured by Ishihara Sangyo Kaisha, Ltd.) as the pigment,and SOLSPERSE 32000 (basic dispersant, manufactured by The LubrizolCorporation) was used as the pigment dispersing resin.

(Pigment Dispersion F)

A pigment dispersion F was obtained in the same manner as in the pigmentdispersion operation for the pigment dispersion A, except that TIPAQUECR-80 (titanium oxide treated with aluminum/silica, primary particlediameter 250 nm, manufactured by Ishihara Sangyo Kaisha, Ltd.) as thepigment, and SOLSPERSE 32000 (basic dispersant, manufactured by TheLubrizol Corporation) was used as the pigment dispersing resin.

(Pigment Dispersion G)

A pigment dispersion C was obtained in the same manner as in the pigmentdispersion operation for the pigment dispersion A, except that TIPAQUECR-60-2 (titanium oxide treated with aluminum/polyalcohol, primaryparticle diameter 210 nm, manufactured by Ishihara Sangyo Kaisha, Ltd.)as the pigment, and SOLSPERSE 32000 (basic dispersant, manufactured byThe Lubrizol Corporation) was used as the pigment dispersing resin.

(Pigment Dispersion H)

A pigment dispersion H was obtained in the same manner as in the pigmentdispersion operation for the pigment dispersion A, except that TIPAQUEPF-690 (titanium oxide treated with aluminum/silica/polyalcohol, primaryparticle diameter 210 nm, manufactured by Ishihara Sangyo Kaisha, Ltd.)as the pigment, and SOLSPERSE 32000 (basic dispersant, manufactured byThe Lubrizol Corporation) was used as the pigment dispersing resin.

(Pigment Dispersion I)

A pigment dispersion I was obtained in the same manner as in the pigmentdispersion operation for the pigment dispersion A, except thatKRONOS2064 (titanium oxide treated with aluminum/organic substance,primary particle diameter 200 nm, manufactured by KRONOS InternationalInc.) as the pigment, and SOLSPERSE 32000 (basic dispersant,manufactured by The Lubrizol Corporation) was used as the pigmentdispersing resin.

(Pigment Dispersion J)

A pigment dispersion J was obtained in the same manner as in the pigmentdispersion operation for the pigment dispersion A, except thatKRONOS2190 (titanium oxide treated with aluminum/zirconium/organicsubstance, primary particle diameter 200 nm, manufactured by KRONOSINTERNATIONAL Inc.) as the pigment, and SOLSPERSE 32000 (basicdispersant, manufactured by The Lubrizol Corporation) was used as thepigment dispersing resin.

(Pigment Dispersion K)

A pigment dispersion K was obtained in the same manner as in the pigmentdispersion operation for the pigment dispersion A, except thatKRONOS2310 (titanium oxide treated withaluminum/silica/zirconium/organic substance, primary particle diameter200 nm, manufactured by KRONOS INTERNATIONAL Inc.) as the pigment, andSOLSPERSE 32000 (basic dispersant, manufactured by The LubrizolCorporation) was used as the pigment dispersing resin.

The compositions of the pigment dispersions A to K prepared as describedabove are summarized in Table 1. “DPGDA” listed in Table 1 representsdipropylene glycol diacrylate; “VEEA” represents vinvloxydiethyleneglycol acrylate (=vinyloxyethoxyethyl acrylate). The solid content(pigment content) in each of the pigment dispersions A to K is 50% byweight with reference to the total weight of the pigment dispersion.

TABLE 1 Pigment Primary particle Pigment diameter dispersion Productname (nm) Surface treatment Pigment dispersant Monomer A TIPAQUE CR-60210 Aluminum DISPERBYK-111 DPGDA B1 TIPAQUE CR-60 210 Aluminum SOLSPERSE32000 DPGDA B2 TIPAQUE CR-60 210 Aluminum SOLSPERSE 32000 VEEA C1TIPAQUE PF-726 210 Aluminum/silica SOLSPERSE 32000 DPGDA C2 TIPAQUEPF-726 210 Aluminum/silica SOLSPERSE 32000 VEEA D TIPAQUE A-220 160Aluminum SOLSPERSE 32000 DPGDA E TIPAQUE CR-58 280 Aluminum/silicaSOLSPERSE 32000 DPGDA F TIPAQUE CR-80 250 Aluminum/silica SOLSPERSE32000 DPGDA G TIPAQUE CR-60-2 210 Aluminum/polyalcohol SOLSPERSE 32000DPGDA H TIPAQUE PF-690 210 Aluminum/silica/polyalcohol SOLSPERSE 32000DPGDA I KRONOS2064 200 Aluminum/organic substance SOLSPERSE 32000 DPGDAJ KRONOS2190 200 Aluminum/zirconium/organic substance SOLSPERSE 32000DPGDA K KRONOS2310 200 Aluminum/silica/zirconium/organic substanceSOLSPERSE 32000 DPGDA

2. Preparation of Ink Examples 1 to 32, Comparative Examples 1 to 5

According to the recipe of the constituents listed Tables 2 to 4, othercomponents listed in the table were added to each of the pigmentdispersions sequentially from the top under stirring, and mixed untilthe polymerization initiator dissolved. Thereafter, the mixture obtainedwas filtered through a 1-μm membrane filter, and coarse particles wereremoved to obtain white inks.

The white inks thus obtained were evaluated for various propertiesaccording to the following procedure. The results are shown in Tables 2to 4.

(Particle Diameter Measurement)

The white inks thus obtained were measured for the pigment particlediameter using a dynamic light scattering particle diameter distributionanalyzer Nanotrac UPA (manufactured by Nikkiso Co., Ltd., model number:UPA-EX150). For the measurement, ethyl acetate was used as themeasurement solvent, and the absorption mode was particle permeability.Measurement was carried out on the precondition that the particle shapeis nonspherical, and the value when the weight cumulative distributionis 50% was used as the weight average particle diameter.

(Evaluation of Decap Property)

Using OnePass JET (manufactured by Tritek Co., Ltd.) as the printingapparatus and KJ4A (manufactured by Kyocera Corporation) as the ink-jethead, the evaluation was carried out according to the followingprocedure. Firstly, the ink was charged into the ink-jet head, a nozzlecheck pattern was printed, and the ejection of ink from all the nozzleswas confirmed. Thereafter, the apparatus was allowed to stand forpredetermined time (2 hours) without nozzle protection such as capping.After the lapse of the defined time, a nozzle check pattern was printed,and the number of nozzle void was confirmed. The evaluation criteria areas follows.

A: No occurrence of nozzle void.

B: Nozzle void is less than five.

C: Nozzle void is five or more.

(Evaluation of Hiding Property)

Using the above-described apparatus used for the evaluation of decapproperty, solid printing was carried out on a transparent PET film (basematerial thickness 100 μm) to form an ink coating film having a filmthickness of 6 μm. Subsequently, ultraviolet rays were applied using aUV lamp to cure the ink coating film, and thus a measurement sample of aprinted material was prepared. The measurement sample was measured forthe L* values of the L*a*b* colorimetric system on a black plate (L*value 14) and a white plate (L* value 95). From the L* value thusobtained, the “hiding rate”defined by formula described below wascalculated, and the hiding property was evaluated.

Hiding rate (%)=(L* value measured on black plate)/(L* value measured onwhite plate)×100

The evaluation criteria are as follows.

A: Hiding rate is 75% or more.

B: Hiding rate is 70% or more and less than 75%.

C: Hiding rate is less than 70%.

(Evaluation of Storage Stability)

The measurement of the ink viscosity was carried out using an E typeviscometer (manufactured by Toki Sangyo Co., Ltd.). The measurementconditions are as follows: the viscosity was measured at 25° C. threeminutes after adjusting the rotation speed to the value appropriated tothe measurement. Storage stability was evaluated from the rate ofviscosity change between the ink viscosity stored at 60° C. for one weekand the ink viscosity before storage. The evaluation criteria are asfollows. The rating “C” or higher was judged as practical level.

A: The rate of viscosity change is less than 5%.

B: The rate of viscosity change is 5% or more and less than 10%.

C: The rate of viscosity change is 10% or more and less than 15%.

D: The rate of viscosity change is 15% or more.

(Evaluation of Ejection Property)

Printing was carried out using the head (KJ4A) manufactured by KyoceraCorporation, the condition of ink ejection was observed by stroboscopicphotographing, thereby evaluating the frequency properties. The waveformwas the Fire 1 mode. For the evaluation, the manner of splitting ofdroplets was observed at the starting of ejection when the frequency waschanged to 5 and 20 kHz, and after continuous ejection for 10 minutes.The manner of splitting of droplets at the points of 1 mm and 2 mm afterejection was observed. The ink is preferably stable without splitting ofdroplets. The evaluation criteria are as follows. The rating “B” orhigher was judged as practical level.

A: Droplets are continuous until 2 mm without splitting, and stable.

B: Droplets are split at 1 mm, and coalesce at 2 mm, or the manner ofsplitting markedly changes from the early stage.

C: Droplets are split at 1 mm, and do not coalesce even at 2 mm;ejection failure occurs in the early stage or 10 minutes after.

(Hue Evaluation)

Hue evaluation was carried out using the measurement samples prepared inthe evaluation of hiding property. These measurement samples weremeasured on a black plate. From the L*a*b* values thus obtained, the huewas evaluated according to the following evaluation criteria.

A: The b* value is less than −5.

B: The b* value is −5 or more and less than −3.

C: The b* value is −3 or more.

TABLE 2 Example 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 DispersionDispersion A B1 B1 B1 C1 C1 C1 C1 C1 C1 C1 C1 C1 B1 B1 B2 B2 typeContent 36 36 44 52 36 44 52 44 44 44 44 44 44 44 44 44 44 Monomer DPGDA47 47 39 31 47 39 31 9 9 9 9 9 9 24 9 14.9 4.9 VCAP 5 5 5 5 5 5 5 5 5 55 5 5 5 5 5 5 PEA 30 LA 30 VEEA 30 30 30 30 15 30 241 341 Initiator TPO9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 Irg819 9 9 BMS 2 2 Irg907 2 2 2 2 2 2 2 22 2 2 2 2 2 2 Solvent DEDG Surface UV3510 1 1 1 1 1 1 1 1 1 1 1 1 1 1 11 1 conditioner Evaluation Particle 210 220 235 240 210 220 230 215 225220 220 220 220 235 235 240 245 diameter (nm) Decap B A B B A A A A A AA A A B A A B property Hiding B B A A B A A A A A A A A A A A A propertyStability C B B C A A A A A A A A A B B C C Ejection B B B B B B B B B AA A A B A A A property Hue B B B B B B B B B B B A A B B B B

TABLE 3 Example 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 DispersionDispersion C1 C2 C2 G G H H C1 C1 I I J J K K type Content 44 44 44 3644 36 44 44 44 36 44 36 44 36 44 Monomer DPGDA 24 14.9 4.9 47 39 47 3938 33 47 39 47 39 47 39 VCAP 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 PEA LA VEEA15 24.1 34.1 Initiator TPO 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 Irg819 BMSIrg907 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 Solvent DEDG 1 6 Surface UV3510 1 11 1 1 1 1 1 1 1 1 1 1 1 1 conditioner Evaluation Particle diameter (nm)220 230 230 215 235 215 230 230 240 210 230 200 225 205 215 Decap A A AA B A A A B A B A B A A property Hiding A A A B A B A A A B A B A B Aproperty Stability A A B A A A A A A A A A A A A Ejection A A A B B A BA A B B B B A B property Hue B B B B B B B B B B B B B B B

TABLE 4 Comparative Example 1 2 3 4 5 Dispersion Dispersion C1 C1 D E Ftype Content 24 66 44 44 44 Monomer DPGDA 59 17 39 39 39 VCAP 5 5 5 5 5PEA LA VEEA Initiator TPO 9 9 9 9 9 Irg819 BMS Irg907 2 2 2 2 2 SolventDEDG Surface UV3510 1 1 1 1 1 conditioner Evaluation Particle 240 210160 310 280 diameter (nm) Decap A C B C C property Hiding C A C A Aproperty Stability A D C C B Ejection B B B C B property Hue C B C C C

Details about the abbreviations listed in Tables 2 to 4 are as follows.

DPGDA: dipropylene glycol diacrylate

VCAP: N-vinyl caprolactam

PEA: phenoxyethyl acrylate

LA: lauryl acrylate

VEEA: vinyloxydiethylene glycol acrylate

TPO: Lucirin TPO (manufactured by BASF, 2,4,6-trimethylbenzoyldiphenylphosphine oxide)

Irg819: Irgacure819 (manufactured by BASF, bis(2,4,6-trimethylbenzoyl)phenylphosphine oxide)

BMS: Speedcure BMS (manufactured by Lambson, 4-benzoyl-4′-methyldiphenylsulfide)

Irg907: Irgacure907 (manufactured by BASF,2-methyl-1-(4-methylthiophenyl)-2-morpholinopropan-1-one)

UV3510: BYE UV3510 (manufactured by BYK-Chemie, silicone surfaceconditioner)

As seen by Tables 2 and 3, when the content of titanium oxide used asthe white pigment and the weight average particle diameter of titaniumoxide are within the predetermined range defined in the presentinvention, good results are obtained in all the desired properties suchas printing stability and hiding property. In contrast, as seen by Table4, it was difficult to satisfy all the desired properties in ComparativeExample 1 to 5, which do not satisfy the content of titanium oxide andthe weight average particle diameter of titanium oxide defined in thepresent invention.

1. An ink, comprising titanium oxide, a pigment dispersant, and apolymerizable compound, wherein a content of the titanium oxide is from19 to 30% by weight with reference to a total weight of the ink, andwherein a weight average particle diameter of the titanium oxide is from180 to 250 nm.
 2. The ink of claim 1, wherein the titanium oxide istreated with a surface-treating agent.
 3. The ink of claim 2, whereinthe titanium oxide has hydroxyl groups on treated surface thereof. 4.The ink of claim 1, wherein the titanium oxide is titanium oxide whosesurface is treated with at least silica.
 5. The ink of claim 1, whereinthe pigment dispersant comprises a basic dispersant.
 6. The ink of claim1, wherein the polymerizable compound comprises at least one of thepolymerizable compounds of formulae (1) and (2):

wherein R1 and R2 are each independently a hydrogen atom or a methylgroup, R3 is a hydrogen atom or an alkyl group with 1 to 10 carbonatoms, and n is an integer of 1 to 10,

wherein R1 is a hydrogen atom or a methyl group, R3 is a hydrogen atomor an alkyl group with 1 to 10 carbon atoms, and m is an integer of 1 to10.
 7. The ink of claim 1, further comprising a polymerizationinitiator, wherein the polymerization initiator comprises at least oneselected from the group consisting of 2,4,6-trimethylbenzoyl diphenylphosphine oxide, bis(2,4,6-trimethylbenzoyl)phenylphosphine oxide, and4-benzoyl-4′-methyldiphenyl sulfide.
 8. The ink of claim 1, furthercomprising an organic solvent, wherein a content of the organic solventis from 0.01 to 5% by weight with reference to the total weight of theink.
 9. The ink of claim 1, wherein the white ink is suitable forprinting on a single-pass ink-jet printing system.
 10. The ink of claim1, wherein the titanium oxide is present in the ink in an amount of from19 to 25% by weight with reference to the total weight of the ink. 11.The ink of claim 1, wherein the titanium oxide is titanium oxide whosesurface is treated with an aluminum compound and silica.